1. Field of the Invention
The present invention is directed to a two-stage reciprocating compressor and the application of such compressors to heating and air conditioning systems and methods. One aspect of the invention is concerned with stabilizing such compressors when the stroke length of one or more reciprocating pistons is switched to change the compressor capacity. Another aspect is the improved throw blocks and lubrication systems for such compressors. Another aspect of the invention concerns a unique electrical circuitry for operating the crankshaft drive motor of the compressor, whereby reversal of the motor for reducing or eliminating the throw takes the motor off the normal run winding and places it on a more efficient winding of reduced current capacity, in particular, the start winding.
Another aspect of the invention is the application of two-stage compressors of the present invention to novel air conditioning and/or heat pump systems and methods. Preferably, such systems include additional two-stage components.
2. Background
The present invention is directed to two stage reciprocating compressors, including scotch yoke compressors such as shown in U.S. Pat. No. 4,838,769. In such machines the reciprocating motion of the pistons is effected by the orbiting of crankpins which are attached to said pistons by connecting rods or other connecting structures having bearings rotatably mounted on said crankpins.
The compressors of the present invention include gas compressors, especially multi-cylinder refrigerant compressors, in which the connecting rod bearing of at least one piston is mounted on an eccentric cam rotatably mounted on the crankpin. This cam angularly adjusts upon reversal of the crankshaft drive motor and the crankshaft to switch to either a lengthened or shortened crankpin throw and piston stroke. Such stroke or throw switching can be engineered to give desired high pressure refrigerant output capacities such that the compressor efficiency can be maintained more easily under varying load requirements.
Throw switching compressors for which the present invention finds particular application are shown and described in U.S. Pat. Nos. 4,479,419; 4,236,874; 4,494,447; 4,245,966; and 4,248,053, the disclosures of which are hereby incorporated herein by reference in their entirety. With respect to these patents the crankpin journal is complex and comprised of an inner and one or more outer eccentrically configured journals, said inner journal being the outer face of the crankpin shaft, and the outer journal(s) being termed "eccentric cams or rings" in these patents, and being rotatably mounted or stacked on said inner journal. The bearing of the connecting rod is rotatably mounted on the outer face of the outermost journal.
In these patents, as in the present invention, all journal and bearing surfaces of the power transmission train of the shiftable throw piston from the crankshaft to the connecting rod are conventionally circular and allow structurally unhindered rotative motion, within design limits, of the outer journal(s) on the inner journal and of the connecting rod bearing on the outermost journal. This rotative motion, in either direction, will, thru the eccentricity of the outer journal surface of the outermost journal relative to its inner bearing surface, shift the radial distance of the orbital axis of the crankpin from the axis of rotation of the crankshaft and thus change the throw of the crankpin and the stroke of the piston.
As described in, e.g., U.S. Pat. No. 4,479,419 and with reference to the structure numbering therein, the angular positioning of the cam 38 on the crankpin 34 is accomplished by providing a pair of drive stops (not numbered in the said 4,479,419 patent, but numbered 58, 60 in the 4,494,447 patent as "end points") which are angularly spaced on a portion of the crankshaft such as the crankpin 34, and a driven dog 48 provided on the cam 38. These stops and the dog are angularly positioned with respect to each other such that upon rotation of the crankshaft in one direction one of the stops will first engage one side of the dog and rotate cam 38 to a first prescribed angular position on the crankpin to produce one piston stroke length. Conversely, reversing the rotation of the crankshaft will terminate this first engagement and cause the other of the stops to rotate to and engage an opposite side of the dog and rotate the cam to a second prescribed angular position on the crankpin to produce another piston stoke length. These angular positions are alternatively characterized herein as "end point(s)" or "dog-stop" junction(s) or "contact junction(s)", all hereinafter termed "junction(s)".
It is noted that at least a portion of the rotation of the cam relative to the crankpin to either of its endpoints can also result from the inertia of the cam or the rotational drag of the strap end bearing of the connecting rod acting on the outer journal surface of the cam.
It is apparent that for a given fixed crankpin throw the maximum possible magnitude of the piston stroke shift will depend on the degree of eccentricity between inner bearing surface and the outer journal surface of the cam. A larger eccentricity will allow an increased or reduced throw depending on the angular position of the cam on the crankpin. Therefore, a properly configured eccentricity will allow the said orbital axis of the crankpin to become coincident with the axis of the crankshaft, thus bringing the crankpin throw and the piston stroke to zero, and thus pacifying the throw, piston and cylinder. It is noted that in this zero stroke or passive mode, the completely pacified piston will remain, theoretically, one half way between its normal top dead center and normal bottom dead center positions during further operation of the compressor in the reduced capacity mode.
It is to be particularly noted, that as mentioned above, all of the journals and bearings involved in this power transmission train are essentially perfectly circular within, of course, modern machining capability, and their rotational contacts with one another are practically frictionless. Thus arises the conundrum that if only one side of the dog is in engagement with a stop at any given time, what is to prevent disengagement of the junction and the consequent rotation of the cam on the crankpin during periods when the cam is being driven by the stop with only minimal force? Such a disengagement could produce a plethora of unplanned piston movements or strokes, which could significantly thwart the effort to maintain maximum compressor efficiency under varying load requirements. Based on a review of the above patents, it apparently was believed that the junction can be maintained simply by the inertia of the cam during such periods.
The U.S. Pat. No. 4,494,447 patent alludes to a destabilizing phenomena, and then only with a glancing mention in column one that gas thrust, piston rod inertia, and centrifugal and gas torque reversal forces contribute to cam instability. In what context and in what relationship however to, e.g., a zero stroke piston mode is not readily apparent from this patent. Also, in column one of that patent it is stated that "--forces which generally tend to prevent the possibility of oscillation are friction forces, various drag loads, and cam inertia forces." This statement appears to be a recitation of those forces which are inherently present, in varying degrees, in all refrigeration compressors, and shed no light on a solution to the instability problem, particularly with respect to a zero piston stroke mode.
This patent then goes on the disclose an actual stabilizing structure constituting its invention, which structure is characterized as aiding in holding the cam in the desired position. This structure comprises end stops 58, 60 which are preferably spaced about 270.degree. apart such that according to the patent a substantial centrifugal force torque "CFT" will develop tending to maintain the stops and dog in contact at the endpoints of the cam rotation, as shown in FIGS. 4 and 5 of the patent. Also as disclosed in this patent, this CFT can be generated by repositioning the center of mass 62 of the cam away from the throw axis which passes thru the crankshaft axis 30a and the crankpin axis 32a, as shown in FIGS. 6 and 7 of the patent.
Applicants have found that with such complex crankpin journals under typical compressor operating conditions, cam inertia alone is ineffective to prevent disengagement, i.e., instability of the junction and the throw shift under the dynamic forces present, even in a theoretically completely pacified cylinder. These compressors have also not been perfected to the extent that they have been used successfully in the marketplace.
In short, while the general concept of two stage compressors is disclosed in the prior art, there exists considerable needs for improvement in these compressors, as to their structure, characteristics, and application in HVAC systems and methods.